Titanium alloys are light in weight and high in strength, and of them, titanium alloys called as near β-type titanium alloys that have a different phase such as the α-phase dispersed in the β-phase are broadly used since they can be hot worked at a temperature lower than the β transformation point and exhibit a high strength.
Of them, Ti-5Al-2Sn-2Zr-4Mo-4Cr is known as having an excellent strength, called as “Ti-17” and is broadly used.
It is also known that β-type titanium alloys or near β-type titanium alloys can increase the strength by being subjected to a heat treatment such as an aging treatment after being shaped. Patent Reference 1 discloses that the tensile strength is improved by subjecting a β titanium alloy to an aging treatment, and discloses that a specimen having a tensile strength of 70 kgf/mm2 (about 690 MPa) improves the tensile strength to 130 kgf/mm2 (about 1270 MPa) by being subjected to an aging treatment, according to a No. 4 specimen in Table 1 of the Patent Reference 1.
Patent Reference 2 discloses that a titanium alloy containing “Ti-17” as a representative component can have an increased strength by setting down the working temperature and the heat treatment temperature.
Meanwhile, in recent years, titanium alloys are required to be increased in strength for further application in various fields or further weight reduction, and sometimes required to have a higher strength than the “Ti-17”. However, the aging treatment is generally carried out by maintaining an object at a temperature of about 500° C. for several hours, and therefore when forming, for example a titanium alloy having a higher strength than the “Ti-17”, it is inevitable to lower the productivity (increase the manufacturing cost) due to the aging treatment. In addition, a special equipment for the aging treatment is required, which results in increase in equipment costs.
That is, conventional near β-type titanium alloys have a problem of making it difficult to obtain near β-type titanium alloys having a higher strength than the “Ti-17” while suppressing the cost increase.
Patent Reference 1: Japanese Patent No. 2669004
Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2001-288518